In crops like corn, uniform seed germination and plant emergence are critical to achieve maximum yield potential. According to The Ohio State University Extension Fact Sheet Tips to Reduce Planter Performance Effects on Corn Yield, “Uneven emergence affects crop performance because competition from larger early-emerging plants decreases the yield from smaller later-emerging plants.” “Emergence delays of 10 days or more usually translate to growth stage differences of two leaves or greater. Therefore, if two plants differ by two leaves or more, the younger, smaller plant is more likely to be barren or produce nubbins.” It is generally known that no yield reduction occurs from late emergence corn as long as the plant emerges within 48 hours of nearby plants. The later a corn plant emerges beyond the 48 hour window, the greater its yield reduction. Research shows that uniform emergence can lead to an average six bushel per acre increase in corn yield.
As compared to corn, field crops such as soybeans and wheat are more effective at making up lost yields for late emerging seedlings. For instance, healthy soybean and wheat plants fill in the space of neighboring weak plants. To a certain degree, healthy soybean and wheat plants tend to produce more grain when nearby plants are behind in growth. Healthy corn plants, on the other hand, are not very effective at recapturing yield lost by nearby stunted plants. For this reason, synchronized plant emergence is critical to maximize corn yield potential and provide all plants with a fair chance at strength and vitality. As depicted in FIG. 1, late emerging seedlings 14 are slow to mature, stunted in height, and develop thinner stalks and smaller ears 18 as compared to healthy, neighboring corn plants 12 with larger ears 16 that produce higher yields. These stunted seedlings 14 steal nutrients from neighboring plants 12 and are considered by farmers to act more like weeds than productive plants. Historically in corn production, more attention has been put on achieving uniform spacing than uniform emergence; however, recent research shows that “uneven emergence has a greater adverse effect on yield than uneven spacing” according to Exapta article Uniform timing of emergence trumps uniform spacing for yield effect. 
Synchronized plant emergence necessitates synchronized seed germination. It is known that late germinating seeds do not catch up in underground growth to earlier germinated seeds because environmental factors that affect growth between germination and emergence are generally the same for nearby plants. In other words, a seed germinated ahead of another will emerge faster because the growth rate between germination and emergence is the same for both seeds. Therefore, delayed germination means delayed plant emergence.
Plant germination depends on a few factors. A first factor is sufficient seed to soil contact. In order for the seed to absorb moisture quickly and uniformly, soil must be firmed around the seed. Seeds set in the bottom of a seed trench at planting ensure uniform seed to soil contact which leads to synchronized germination. Seed to soil contact can be maximized by proper down pressure on individual row units on a planter. A “seed firmer” can be used to improve seed to soil contact by pushing the seed firmly into the bottom of a seed trench after the planter dispenses seed into the trench. The “seed firmer” tool can improve seed germination by improving seed to soil contact in loose soil, but it can lose its effectiveness when soil voids caused by clods at the bottom of seed trench occur.
What is needed is an on-the-go means to detect soil voids in the seed trench. An on-the-go seed to soil contact sensor can be used to adjust planting depth deeper to reach solid formed soil. It could also be used to adjust a trash cleaner tool or tillage tool mounted ahead of the row to remove or pulverize clods.
A second factor for encouraging synchronized seed germination and plant emergence is adequate soil moisture. Corn seeds must imbibe an adequate amount of moisture to start and complete germination. Adequate soil moisture for corn is most simply defined as enough moisture to swell the seed triggering utilization of starch in the kernel and the emergence of a radical root. The seed must imbibe enough moisture to get root growth to the point the roots can take over supplying the young seedling with nutrients and moisture. Marginal levels of soil moisture from dry soil may cause seeds to germinate and emerge late relative to nearby seeds. Uneven soil moisture throughout the seed zone is a primary cause of uneven germination and emergence, the results of which can be yield loss. Calculating the overall soil moisture level of a field prior to planting is difficult, as soil moisture varies throughout the field and depends on several factors such as topography, weather conditions, tillage patterns, soil profile, and uneven seeding depth. Soil typically retains moisture in the valleys of a field while drying out faster on hilltops and hillsides. Empirical readings while planting have depicted up to a 2 to 1 difference in soil moisture at the same depth in different areas of the same field. Moreover, soil profiles vary in their ability to hold water. For example, FIG. 2 illustrates a field comparison of moisture levels on the top of a clayous hill 22 versus a valley 24 with loam soil. At the top of the hill 22, there is approximately 9% soil moisture at two inches in depth, while in the valley 24 there is approximately 13% soil moisture at two inches in depth. Thus, it is extremely difficult to determine the overall level of moisture in a field prior to planting.
Soil moisture levels increase as seeds are planted deeper into the soil. Thus, if the soil is dry and no precipitation is predicted in the immediate future, farmers plant seeds deeper into the soil to reach the required moisture levels to initiate germination. Soil moisture meters are known in the art as measuring moisture at different depths of the soil and can be used to help determine seed planting depths. However, commercially available sensors generally require stationary readings in order to operate. As a result, few farmers use soil moisture meters to set planting depths as they lack confidence in the soil moisture reading to achieve synchronous seed germination throughout a field.
Instead of utilizing soil moisture meters, many farmers simply determine planting depth on an ad hoc basis by digging up a planted seed. If soil surrounding the seed feels and appears to contain the required amount of moisture, planting depth is considered satisfactory. Such determinations are often made by merely pinching the soil with the fingers. Soil that sticks together is considered to maintain a satisfactory amount of moisture, while soil that fails to stick together indicates that planting depth needs to be increased. This age-old technique is largely based on past planting experience and is obviously subject to human error. In addition, the number of samples tends to be very limited in size. The bottom line is farmers do not have a good means to account for inconsistent soil moisture levels throughout a field during planting.
Thus, what is needed is a reliable, on-the-go soil moisture sensor to provide real-time readings at planting depth to the farmer while planting. On-the-go soil moisture sensors can be used to manually or automatically adjust planting depth to a depth containing optimum moisture levels for seed germination. On-the-go soil moisture sensors generally are not commercially available despite a few examples utilized in research venues. For example, V. I. Adamchuk et al. “On-the-go sensors for precision agriculture” (March 2004) discloses on-the-go soil moisture sensor research including electrical, electromagnetic, optical and radio metric sensors and methods. Lie et al. “Development of a texture/soil compaction sensor” (1996) incorporated a dielectric-based soil moisture sensor into an instrumented chisel and conducted field tests. Andrade et al. “Evaluation of a dielectric based moisture and salinity sensor for in situ applications” (2001) improved upon Lie's on-the-go sensor by overcoming the interference of temperature and salinity. Gaultney et al. “Development of a Soil Moisture Meter to Predict Corn Seed Planting Depth” (1991) and Weatherly et al. “Automatic Depth Control of a Seed Planter Based on Soil Drying Front Sensing” (1997) discloses automatically controlling planting depth based on the soil moisture readings of an on-the-go sensor. Each aforementioned reference is herein incorporated by reference in its entirety as if set forth fully herein.
On-the-go soil moisture sensors are also beneficial on non-seeder implements like field cultivators, anhydrous applicators, chisel plows, moldboard plows, vertical tillage implements, strip till and other tillage implements. Tilling soil too wet causes soil compaction which restricts root growth and can reduce yield. Soil moisture sensors can be used to adjust tilling depth to avoid soil compaction or to avoid tillage altogether until the field dries out. They could also be used with implements applying fertilizers and pesticides to adjust tillage depth to a soil moisture level that causes the fertilizer or pesticide to work more effectively for the crop. On-the-go soil moisture sensors could also be used to vary the rate of fertilizers or pesticides to make them work more effectively for the crop.
A third factor for encouraging synchronized seed germination and plant emergence is soil temperature. According to the Tips to Reduce Planter Performance Effects on Corn Yield article from The Ohio State University Extension, “The optimum temperature for germination and emergence is 68 degrees F. to 72 degrees F. Emergence occurs in five to six days at these temperatures. Soil temperatures below 50 degrees F. dramatically slow germination and emergence. Individual seeds in a furrow may be subject to different temperature and moisture conditions due to placement.”
Soil temperature decreases the deeper seed is planted. An on-the-go soil temperature sensor can be used on planting implements to adjust planting depth to an optimum temperature level for seed germination. It can also be used with non-seeder implements to adjust tillage depth for the purpose of making fertilizers and pesticides more effective for the crop. It can also be used to vary the rate of fertilizers and pesticides to make them more effective for the crop.
A fourth factor for encouraging synchronized seed germination and plant emergence is proper planting depth. It's already been shown optimum planting depth is dependent on the aforementioned factors; however, sensing the actual planting depth from the top of the soil to the bottom of the seed trench is important on its own accord. For example, plant residue can cause the depth regulation member (e.g. gauge wheels) of a planter row unit to ride up on top of the residue causing a shallower cut seed trench than intended. Gauge wheel load sensors are known in the art to sense when the row unit is cutting a seed trench at the intended depth; however, they don't account for the depth error from plant residue and they can't measure the magnitude of the depth of cut of the seed trench. On-the-go seed trench depth sensors provide feedback for adjusting planting depth to maintain a desired target seed trench depth. They can also provide cutting depth information to non-seeder tillage implements for the purpose of tilling at an intended depth.